Currently the key prop is truncated to 15 or so letters. I've been unable to locate where in the devtools code this happens. There is a truncateText in utils but it does not appear to be called by anyone.
This truncation is intentional to keep nodes from being too wide (which impacts horizontal indentation).
You can hover over the key value to see the full string (as a tooltip) or double-click select it if you want to copy paste the un-truncated value though.